Engineering of an Intersubunit Disulfide Bridge in the Iron-Superoxide Dismutase of Mycobacterium tuberculosis

With the aim of enhancing interactions involved in dimer formation, an intersubunit disulfide bridge was engineered in the superoxide dismutase enzyme of Mycobacterium tuberculosis. Ser-123 was chosen for mutation to cysteine since it resides at the dimer interface where the serine side chain interacts with the same residue in the opposite subunit. Gel electrophoresis and X-ray crystallographic studies of the expressed mutant confirmed formation of the disulfide bond under nonreducing conditions. However, the mutant protein was found to be less stable than the wild type as judged by susceptibility to denaturation in the presence of guanidine hydrochloride. Decreased stability probably results from formation of a disulfide bridge with a suboptimal torsion angle and exclusion of solvent molecules from the dimer interface.